1. Field of the Invention
This invention relates to spectral sensitizers useful in photopolymer applications, for example printing plate and proofing applications. More particularly this invention relates to aminoketone-substituted coumarin sensitizers having tethered tertiary amino groups.
2. Background of the Art
Photoinitiators for polymerization reactions ordinarily have an inherent, but limited range of natural radiation sensitivity. Only when the photoinitiators are stimulated by these particular ranges or wavelengths of sensitivity do the photoinitiators display their desired functional ability. It is not feasible and/or economic to attempt to provide different photoinitiators for each different range of wavelengths to which one would desire a polymerizable system to be sensitive. It is therefore better to find ways of changing the wavelength range of sensitivity for a given photoinitiator. Changes in sensitivity can be made by structural alteration of the photoinitiator, but a more accepted and more easily performed method is spectral sensitization of the photoinitiator. This procedure is commonly practiced by finding dyes which absorb radiation in the desired range of wavelengths and which activate the photoinitiator after absorption of the radiation. For each photoinitiator system it is difficult to predict which specific dyes will be able to effectively act as efficient sensitizers. Even though some dyes may work with more than one photoinitiator (or other photosensitive systems such as silver halides, tetrahydracarbyl borates, etc.), there is seldom any certainty that a compound or class of compounds will work with a particular photoinitiator.
Since many spectral sensitizing dyes which absorb visible radiation to perform their function, they may be colored and add an undesirable color, tone, or hue to the polymerized material. It is often desirable to provide spectral sensitizing dyes which may be treated to lose their color after they have been used as spectral sensitizers. This can be accomplished in many ways such as thermal bleaching or photobleaching of the dyes.
Sensitizers may also be used which merely increase the speed (sensitivity) of the photoinitiators at those wavelengths to which they are naturally sensitive.
Japanese published applications 63-168403 (Fuji; publication date Jul. 12, 1988) and 63-309502 (Hitachi; publication date Jul. 12, 1988) generically disclose photopolymerizable compositions containing aminoketone and ketocoumarin sensitizing dyes. Neither application describes specific examples which are within the scope of, or demonstrate, the surprising increases in speed associated with the instant invention.
Published EPO application 290,133 (claiming priority to U.S. Ser. No. 034,065 filed Apr. 2, 1987 and assigned to Minnesota Mining and Manufacturing Company) describes aminostyryl dyes in combination with a sensitizer and an electron donor (which may be a tertiary amine). The published EPO application does not describe the advantages derived from linking the tertiary amine to the sensitizer. Compounds of the instant invention are shown in the examples to have higher sensitivity in photopolymer compositions than those of said published application.
Specht et al. (U.S. Pat. Nos. 4,147,552; 4,289,844; and 4,278,751; Jul. 14, 1981) disclose aminoketone-substituted coumarins having a variety of substituents including tertiary amino substituents. These patents do not teach the particular advantage of increased photosensitivity of the compounds of the present invention. The aminoketone-substituted coumarin sensitizers materials of Specht et al. do not fall within the formula of the instant invention.
Adair et al. (U.S. Pat. No. 4,713,312; Dec. 15, 1987) discloses the use of aminoketone-substituted coumarins as sensitizers for photosensitive microcapsules. Those material disclosed are essentially similar to those of U.S. Pat. No. 4,147,552.
U.S. Pat. No. 4,289,844 (Specht et al.) and U.S. Pat. No. 4,247,623 (Guild) disclose aminoketone-substituted coumarin sensitizers in combination with amine activators. Neither patent teaches the principal advantage (i.e., speed) which is achieved when a tertiary amine is covalently tethered to an aminoketone-substituted coumarin sensitizer, instead of being added as a secondary component.